Osmotic and volaemic effects on drinking rate in elasmobranch fish

An increase in drinking rate of two species of marine elasmobranch fish, Scyliorhinus cauicula and Triakis scyllia, acclimated to 80% sea water was observed following the introduction of 100% sea water to experimental tanks. The drinking response in both species was found to be maximal within 6h, and a significant increase was sustained for up to 24h in T. scyllia. Plasma osmolality was significantly increased within 6 h following introduction of 100% sea water, and this increase was principally due to elevated plasma Na+ and Cl-concentrations. Administration of 2 moll(-1) mannitol, 75% sucrose and vehicle (elasmobranch Ringer) did not induce a significant increase or decrease in the drinking rate of S. canicula. However, injection of 20% NaCl was found to decrease drinking rate significantly in S. canicula 60 min after administration. Controlled haemorrhage of approximately 5.7% of total blood volume in S. canicula induced a rapid 36-fold increase in drinking over basal levels. The present study demonstrates a physiological dipsogenesis in response to hypovolaemia in marine elasmobranch fish as part of their overall iso/hyperosmoregulatory strategy.